There has been proposed an exhaust emission purifying apparatus in which a liquid reducing agent or its precursor is injection-supplied to an exhaust emission upstream side of a NOx reduction catalytic converter disposed in an engine exhaust system, so that NOx in the exhaust emission and the reducing agent are subjected to a catalytic-reduction reaction, to thereby reductively purify NOx into harmless components. In the exhaust emission purifying apparatus, when the concentration of the liquid reducing agent or its precursor is changed depending on some factor, there is a possibility that NOx purification efficiency in the NOx reduction catalytic converter is degraded, so that required NOx purification performance may not be obtained. In particular, if a mixing ratio between solute and solvent in the liquid reducing agent or its precursor is improper or if any different type of aqueous solution or water is mixed into the liquid reducing agent or its precursor, the above-mentioned problems are prominently caused. Therefore, as disclosed in Japanese Patent No. 3687915 (Patent literature 1), the applicant of the present invention has proposed a technology for, even if convection is generated in a liquid stored in a reducing agent tank, discriminating with high precision a liquid type of the liquid, by utilizing a concentration sensor which measures the concentration of a liquid reducing agent or its precursor based on temperature rise characteristics of a heating element.